Cupping Problem with Engineered Wood Floors

Engineered wood flooring consists of a thin layer (veneer) of hardwood glued onto a backing. The backing can be plywood or some type of fiberboard. Since the backing is invisible, inexpensive softwood or wood fibers are used. One of the first engineered woods was plywood. Plywood is a very stable panel manufactured by gluing several layers of veneer together and changing the orientation of the growth rings with each layer.

Engineered floors and panels are expected to be at least as stable as a solid piece of wood, and they often are very stable. However, being made from different layers of materials can present a new set of problems. These problems appear mainly, when the moisture is not right to begin with or when the planks are exposed to changes in moisture content.

If the top layer and the backing have the same shrinking tendencies, engineered wood will move just like solid wood. If the top layer and the backing shrink differently, then the panel is more moisture sensitive and more shrinking and warping can be expected.

Engineered Wood Floor Cupping

When the top layer shrinks or expands and the backing does not follow at the same rate, the plank will curl, the panel will delaminate or surface checks appear. Almost every piece of wood inside a home will endure some moisture movement in the course of a year. Weather and changing seasons affect the relative humidity inside a home and consequently the relative humidity affects the wood moisture in floors, panels or furniture. Small cracks appear and disappear with the changing seasons. This is true for solid and engineered wood. Operating an HVAC year round will stabilize the wood.Changes of relative humidity are usually followed by changes in wood moisture:  For example, when the EMC of air is lower than the wood moisture content (dry winter), the surface and eventually the entire piece of wood dries out until the moisture content has reached the EMC.

Solid Wood: When wood is drying out, the surface dries first and shrinks. The wetter core will follow later. Cupping and surface checking can occur. Stresses can build up near the surface and cause the wood to check (small surface splits). If the deformations are severe enough, the structure of the wood can be broken and the defects are permanent. 
Over time, the core will also dry out and the entire piece of wood will have a uniform moisture distribution again. Cupping may disappear and small gaps between the floor planks may be visible.  When an engineered floor dries out, the same drying process occurs as described above. The surface dries first and the veneer shrinks. The veneer will determine how much shrinking occurs. Since the core is usually very stable, the hardwood layer on top is responsible when problems occur:

-When the veneer shrinks equally with the core, you have a very stable product 
-When the veneer shrinks a lot more than the core, you have a product which can develop problems whenever the moisture changes. That engineered floor will only be flat at the manufactured moisture content, and the veneer will not fit the core at any other moisture content.